1. Field of the Invention
The present invention relates to a gain controlled amplifier, and more particularly, to a gain controlled amplifier using active feedback and variable resistance.
2. Description of the Prior Art
In general, a variable gain amplifier is used to receive input signals having different power levels from each other, to amplify them in accordance with each power level and to send them as output signals having a constant power level. Among many methods to control gain, there is a method by which an attenuator is installed in the input or the output of an amplifier. However, it has problems as follows: if the attenuator is installed in the input, the noise characteristics get worse; if the attenuator is installed in the output, the output power characteristics are degraded; if the attenuator is installed between the input and the output, it imposes burden on the previous stage as the attenuation of the attenuator in terms of power characteristics.
A method which controls the bias of an amplifier has an advantage of controlling gain simply, but it has a problem that the power characteristics are degraded when condensing its consumption power to reduce gain. Though using a feedback circuit in amplifier in order to settle the problem, there is also a problem that the power and the matching characteristics are degraded since the load of the output of the amplifier is changed in accordance with the degrees of its feedback when using variable resistance in the feedback circuit. When using a common drain field effect transistor for the feedback circuit and controlling gains by using the gate bias of the field effect transistor, it is possible to prevent the degradation of the power characteristics by changing the load of output since the matching point is changed a little against the change of the gate bias and the isolation characteristics from the drain terminal to the gate terminal are excellent. However, since the power consumption should be provided properly to a common drain field effect transistor used for the power characteristics of signals feedbacked in that structure, the size of resistance value used for the bias of the source is restricted and, therefore, the gain of the amplifier is degraded because of the loss by the resistance.
FIG. 1 illustrates the circuit diagram of a variable gain amplifier using an active feedback according to the conventional art. As shown in FIG. 1, the circuit of the variable gain amplifier using the active feedback according to the conventional art is as follows:
Signals input to an signal input terminal 100 pass through an input matching circuit 101, are amplified in the first field effect transistor 104, pass through an output matching circuit 105 and are output to a signal output terminal 122. However, a part of signals output to the drain of the first field effect transistor 104 are transmitted into the gate of the second field effect transistor 111, are amplified in the second field effect transistor 111 in accordance with the gate bias point and are feedbacked to the gate of the first field effect transistor 104 through the source. At this time, since the phases of the original signal transmitted to the gate of the first field effect transistor 104 and the feedbacked signal transmitted through the second field effect transistor 111 are delayed 180.degree., the gain of the amplifier is reduced in proportion to an increase of the amount of signals transmitted by feedback. However, it is required to provide the power consumption properly to the drain common field effect transistor in order to prevent the degradation of power characteristics of the feedbacked signal in the power characteristics of the amplifier. Consequently, it generates problems that the size of the resistance value of a resistor used for the bias in the source of the common drain field effect transistor is limited and that the gain of the amplifier is degraded because of the loss by the resistance.